Electrode for semiconductor chip and semiconductor chip with the electrode

ABSTRACT

In an n-type semiconductor layer that contains gallium (Ga), contact resistance is to be suppressed at a low level. An n-side electrode is provided on a surface of the n-type semiconductor layer containing Ga. The electrode includes a metal layer having a Ga content of equal to or more than 1 at % and equal to or less than 25 at %. The metal layer is disposed in contact with the n-type semiconductor layer.

This application is based on Japanese patent applications No.2008-150470 and 2009-104732, the contents of which are incorporatedhereinto by reference.

BACKGROUND

1. Technical Field

The present invention relates to an electrode for a semiconductor chip,a semiconductor chip with the electrode, and a method of manufacturingsuch semiconductor chip.

2. Related Art

For a so-called next-generation DVD such as a Blu-ray disk or aHigh-Definition Digital Versatile Disc (hereinafter, HD-DVD), laser beamof 405 nm or so in wavelength is employed for retrieving data recordedon the disk surface and writing new data thereon. For such purpose, asemiconductor laser based on a semiconductor crystal predominantlycontaining gallium nitride (hereinafter, GaN) is employed as the lightsource of the laser beam.

In a semiconductor laser of a shorter oscillation wavelength, the activelayer has a wider band gap and hence a larger built-in potential, andtherefore a higher voltage has to be applied for supplying a current.Also, the semiconductor laser typically has a double-hetero structurewhich encourages efficient reunion of electrons and holes in the activelayer. Accordingly, the semiconductor laser includes a clad layer havinga wider band gap than the active layer.

In a semiconductor in general, the higher the band gap is, the higherthe electric resistance becomes. Therefore, normally the semiconductorlaser of the shorter wavelength has the higher electric resistance inthe clad layer in the same way.

Further, the semiconductor laser includes metal electrodes disposed incontact with the p-type semiconductor and the n-type semiconductorrespectively. The contact resistance between the electrode and each ofthe semiconductors tends to be higher, in the semiconductor having thewider band gap. For such reasons, higher working voltages are requiredfor supplying the same current, in the ascending order of the laser fora Compact Disc (hereinafter, CD) (wavelength 780 nm), for a DVD(wavelength 650 nm), and for a next-generation DVD (wavelength 405 nm).

A recording and reproducing apparatus for the next-generation DVD alsoincludes the lasers for reading and writing in conventional media suchas the CD and the DVD, in addition to the laser for the next-generationDVD such as the blue ray disk and the HD-DVD. Among those lasers, theone for the next-generation DVD requires a prominently higher workingvoltage, and therefore it is desirable to lower the working voltage asmuch as possible, to thereby employ a common power source. For thispurpose, research and development of the metal electrode arepersistently being made, for attaining a lower contact resistance withthe p-type semiconductor layer and the n-type semiconductor layer.

Japanese Laid-open patent publication No. H09-8407 proposes a GaN-basedsemiconductor light emitting element in which the n-side electrode isconstituted of Ti/Nb/Au layered in this order from the side of thesemiconductor chip.

Also, Japanese Laid-open non-patent publication No. 2005-26291 proposesa GaN-based semiconductor light emitting element in which the n-sideelectrode is constituted of Pd/Mo/Au layered in this order from the sideof the semiconductor chip.

The non-patent document (“Interfacial reactions of Ti/n-GaN contacts atelevated temperature”, Lu C. J. et al, Journal of Applied Physics, 94,1, (2003) pp. 245-253) provides observational data of interfacialreaction that takes place on the contact interface between Ti and then-type GaN when heated up to 700° C. or so.

The foregoing techniques, however, still have a room for improvement inthe following aspects.

Regarding the n-type GaN-based semiconductor, it is known that arelatively low contact resistance can be attained by employing a metalof a small work function such as Ti, V, or Nb as the contact electrodedisposed in contact with the n-GaN layer. In the semiconductor laser,however, the contact resistance may deteriorate from the initial valueowing to current supply or heating.

Moreover, Lu C. J. et al reports that heating provokes diffusion of Gafrom the n-GaN layer toward the metal electrode. In the GaN a Ga defectis produced after the diffusion of Ga, and such Ga defect often acts asa p-type dopant. Accordingly, the contact resistance between the n-GaNlayer and the metal electrode is increased. The increase in contactresistance due to supplying current is also partly because of theemergence of the defect originating from the Ga diffusion.

According to Lu C. J. et al, the heating also provokes diffusion of thenitrogen atom in the n-GaN layer. The N defect tends to act as an n-typedopant, and hence the contact resistance between the n-typesemiconductor layer and the electrode may be expected to decrease. Inpractical operation of the semiconductor device, however, the contactresistance is actually prone to deteriorate because the emergence of theGa defect or void exerts a greater effect toward increasing theresistance.

Such drawback is an issue to be resolved in common to the semiconductorchips that include the contact electrode disposed on the n-typesemiconductor layer in contact therewith, without limitation to thenitride semiconductor chip including the n-GaN layer.

SUMMARY

In one embodiment, there is provided an electrode for a semiconductorchip, to be formed on a surface of an n-type semiconductor layer thatcontains gallium (hereinafter, Ga), comprising a metal layer having a Gacontent of equal to or more than 1 at % and equal to or less than 25 at%, wherein the metal layer is disposed in contact with the n-typesemiconductor layer.

In another embodiment, there is provided a semiconductor chip comprisingthe foregoing electrode.

In still another embodiment, there is provided a method of manufacturinga semiconductor chip, comprising preparing an n-type semiconductor layerthat contains Ga, forming an n-side electrode on a surface of the n-typesemiconductor layer, wherein the forming the n-side electrode includesdepositing a Ga-containing metal layer on the surface of the n-typesemiconductor layer, and employing a metal material having a Ga contentof equal to or more than 1 at % and equal to or less than 25 at %, tothereby form the metal layer.

In the semiconductor chip thus constructed, the metal material having aGa content of equal to or more than 1 at % and equal to or less than 25at % is disposed in contact with the Ga-containing n-type semiconductorlayer. An impurity in a semiconductor diffuses, according to the Fick'slaw, at a velocity proportional to the concentration gradient. In otherwords, atoms diffuse from a high-concentration region toward alow-concentration region in a solid. Accordingly, providing Ga inadvance in the electrode allows preventing the diffusion of Ga from thesemiconductor layer to the electrode. Adjusting the Ga content in therange of 1 at % to 25 at % enables properly decreasing the contactresistance of the electrode, as well as forming the metal layer havingthe desired Ga content under accurate control. Such arrangement enables,consequently, suppressing the deterioration in contact resistanceoriginating from the operation of the device and thermal history.

Thus, the present invention enables attaining a low contact resistancewith a Ga-containing n-type semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description ofcertain preferred embodiments taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of an electrode according to a firstembodiment;

FIG. 2 is a diagram for explaining a method of manufacturing theelectrode according to the first embodiment;

FIG. 3 is a graph for explaining the effect of the electrode accordingto the first embodiment;

FIG. 4 is a cross-sectional view of a semiconductor laser according to asecond embodiment;

FIG. 5 is a cross-sectional view for explaining a working of thesemiconductor laser according to the second embodiment; and

FIG. 6 is a cross-sectional view of a semiconductor laser according to athird embodiment.

DETAILED DESCRIPTION

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purposes.

Hereunder, embodiments of the present invention will be describedreferring to the drawings. In all the drawings, same constituents aregiven the same numeral, and the description thereof will not berepeated.

First Embodiment

FIG. 1 is a cross-sectional view of an electrode according to thisembodiment. The electrode is an n-side electrode 110 formed on a surfaceof an n-type semiconductor layer 201 that contains Ga. The n-sideelectrode 110 includes a metal layer 202 containing equal to or morethan 1 at % and equal to or less than 25 at % of Ga. The metal layer 202is located in contact with the n-type semiconductor layer 201.

To be more detailed, the n-side electrode 110 has a multilayer structureincluding the metal layer 202 and a gold-plating layer 205 constitutedof gold (hereinafter, Au) and formed over the metal layer 202. Thegold-plating layer 205 is an uppermost layer of the multilayerstructure.

Between the metal layer 202 and the gold-plating layer 205, a Pt layer203 and a Au layer 204 are stacked in this order over the metal layer202. Over the surface of the Au layer 204, the gold-plating layer 205 isprovided.

The n-type semiconductor layer 201 may be constituted of a III-Vcompound such as GaN. In particular, a nitride semiconductor layer dopedwith an impurity such as Si or Ge, for example GaN, InGaN, or AlGaNfacilitates forming a desirable ohmic contact. Above all, adopting GaNenables attaining an excellent ohmic contact.

The metal layer 202 of the n-side electrode may be constituted of ametal material containing equal to or more than 1 at % and equal to orless than 25 at % of Ga. Any metal material may be adopted provided thatit is stable in the air and has a suitable melting/boiling point as anelectrode. Ga content of the metal layer 202 is analyzed by using AugerElectron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS),Electron Probe Micro Analysis (EPMA), or the like. In these analysismethods, the element composition of the membrane surface is analyzed.Therefore, Ga content of interior position of the metal layer 202 enableto be analyzed while the metal layer 202 is etched by using sputteringmethod.

It is preferable to employ a material having a small work function as ahost metal. In the case where the n-side electrode 110 is provided on achip in which current runs from the p-side to the n-side, and where themetal material constituting the n-side electrode has a large workfunction, the contact between the n-side electrode 110 and the n-typesemiconductor layer 201 forms a Schottky barrier junction, which impedesthe current to run. Although the work function of the metal material maybe appropriately set so as to be smaller than the electron affinity ofthe n-type semiconductor layer 201, it is preferable to set the workfunction at 5 eV or lower. A lower limit is not specifically determined,however it is preferable from a practical viewpoint that the workfunction is not lower than 3.0 eV.

The host metal predominantly constituting the metal material may beselected from those having a work function of 5 eV or lower, for exampleat least one of Ti, Nb, Al, Ta, V, and Hf. These metal materials arepreferable because of the lower work function, which facilitatesproviding a desirable ohmic characteristic. Especially, Ti provides anexcellent ohmic characteristic. The work function of the respectivemetals is as follows: Ti 4.4 eV, Nb 4.3 eV, Al 4.2 eV, Ta 4.3 eV, V 4.3eV, and Hf 3.9 eV (Kochi University of Technology, Electronic andPhotonic System Engineering Course, Graduate Research Report 2002, p. 4(http://www.kochi-tech.ac.jp/library/ron/2002/2002e1e/10 30191.pdf).

In the case of adopting Ti for constituting the metal layer 202, acompound with Ga, such as Ti₃Ga or Ti₂Ga, may be employed. The formerhas the smaller Ga content, which is 25 at %.

Adjusting the Ga content of the metal layer 202 to be 25 at % or lowerpermits presence of a region solely occupied with the host metal in themetal bulk, thereby suppressing the contact resistance at a low level.It is undesirable to compose the entirety of the metal layer 202 fromthe compound containing the host metal and the Ga as component elements,because the properties become substantially different. Setting the upperlimit of the Ga content at 25 at % allows preventing forming theentirety of the metal layer with the compound. In the case where Ti isthe host metal, it is desirable that the Ga content is adjusted so as tobe 10 at % or lower through taking into consideration thecrystallization performance of Ti.

Providing 1 at % or more of Ga in the metal layer 202 enablessuppressing the contact resistance at a low level. The Ga content of 1at % or more is also advantageous in processability and controllability.

Forming the metal layer 202 in a thickness of 100 Å or more facilitateseffectively decreasing the contact resistance.

The Pt layer 203 serves as a barrier metal that inhibits Au fromdiffusing as far as the interface of the semiconductor. In the case ofalloying the n-side electrode 110 at 400° C. or higher, it is preferableto form the Pt layer 203. In the case where the metal layer 202 issufficiently thick, the Pt layer 203 may be omitted. This is because thesufficient thickness of the metal layer 202 prevents the diffusion of Auto the semiconductor interface, although the Pt layer 203 is absent.

The gold-plating layer 205 serves to enhance the wire bondingperformance to the n-side electrode 110, and to alleviate a damage thatthe wire bonding process may impose on the n-type semiconductor layer201.

A method of manufacturing the n-side electrode 110 according to thisembodiment will now be described. To start with, the n-typesemiconductor layer 201 containing Ga is prepared. The nitridesemiconductor generally turns to the n-type because nitrogen voids areformed in the crystal without being doped, however doping an impuritysuch as Si or Ge during the growth makes a more suitable n-type nitridesemiconductor. Further, the GaN-based compound semiconductor may begrown by gas phase epitaxy such as metal-organic chemical vapordeposition (MOCVD), metal-organic vapor phase epitaxy (MOVPE), orhydride vapor phase epitaxy (HVPE).

Then the n-side electrode 110 is formed on the n-type semiconductorlayer 201 prepared as above. A process of depositing the metal layer 202over the surface of the n-type semiconductor layer 201 will be describedin details.

The deposition methods of the metal layer 202 include a RF sputteringprocess and a vapor deposition process. Each of those may be executed asfollows.

(1) RF Sputtering Process

The crystal of a host metal (for example, Ti crystal) containing anappropriate amount of Ga is sintered in advance. The deposition by theRF sputtering process is performed with the metal sintered compact as atarget material. In the case where the host metal is solid at roomtemperature, setting the Ga content at 25 at % or lower makes thesintered compact a solid at room temperature. By this method, a chemicalreaction or phase shift between the chemical species barely takes place,and the target material is clusterized in vacuum by ion-etching andstacked on the sample. Accordingly, the structure of the deposited metallayer 202 becomes substantially the same as that of the target material.

(2) Vapor Deposition Process

The vapor deposition process basically includes depositing a chemicalspecies evaporated from a source material owing to heating for theprocess, over the sample.

Since the phase shift to gas takes place in the deposition process, thestructure of the deposited layer becomes different from that of thesource, because of the difference in the vapor pressure of therespective chemical species. Besides, employing a mixture of differentmetals as the source may provoke bumping, because of the difference inboiling point between the metals. Accordingly, it is preferable to adopta co-deposition process in this embodiment.

Specifically, as shown in FIG. 2, a melting pot 402 containing Ga andanother melting port 404 containing the host metal (for example, Ti) areprepared, and a resistance heater 401 and an electronic beam 403 areemployed, to evaporate Ga and Ti respectively. The evaporated Ga and Tistick to the n-type semiconductor layer 201 and are deposited thereon,to thereby form the metal layer 202.

A reason of employing the resistance heater 401 for Ga is that Ga isliquid at room temperature. The resistance heating, which heats theentirety of the source, is generally more suitable for a liquid than theelectronic beam which heats a local portion, because the former providesa more stable deposition rate. However, the electronic beam may also beemployed if careful operation can be assured. Properly controlling thepower of the resistance heater 401 for Ga and of the electronic beam 403for Ti enables obtaining the metal layer 202 of a predetermined Gacontent.

Over the metal layer 202 thus deposited, the Pt layer 203 and the Aulayer 204 are sequentially deposited. The RF sputtering process may alsobe employed, for depositing the Pt layer 203 and the Au layer 204. Thenthe gold-plating layer 205 is formed on the Au layer 204 and anunnecessary portion is removed, for example by a milling process, sothat the n-side electrode 110 is completed.

It is to be noted that the n-side electrode 110 according to thisembodiment is applicable to various semiconductor chips that include ann-type semiconductor layer. Examples of such semiconductor chip includea semiconductor laser, a light emitting diode, and an electronic device.

The sample-making process of the n-side electrode 110 shown in FIG. 1and the evaluation thereof will be described hereunder. First, an n-GaNlayer was prepared as the n-type semiconductor layer 201. Then a TiGalayer was deposited over the surface of the n-GaN layer in a thicknessof 300 Å, so as to serve as the metal layer 202. The deposition wascarried out by a sputtering process. The content of Ga in the TiGa layerwas adjusted to be 1% in atomic concentration. The content of Ga in theTiGa layer was analyzed by using XPS method. On such TiGa layer, the Ptlayer 203 and the Au layer 204 were sequentially deposited, both in athickness of 150 Å, by a RF sputtering process. The Au was plated in athickness of 205 Å, and an unnecessary portion was removed, for exampleby a milling process, so that the n-side electrode 110 was completed.

The contact resistance with the n-GaN layer was evaluated through aTransmission Line Model (TLM) method. As a result, the contactresistance in the order of 10⁻⁵Ωcm² was obtained. Also, after alloyingat 800° C. for 15 minutes in a nitrogen atmosphere, the contactresistance in the order of 10⁻⁵ Ωcm² was obtained as well.

The foregoing embodiment provides the following advantageous effects. Inthis embodiment, the metal layer 202, constituted of the metal materialcontaining equal to or more than 1 at % and equal to or less than 25 at% of Ga, is provided in contact with the n-type semiconductor layer 201containing Ga. An impurity in a semiconductor diffuses in proportion tothe concentration gradient, according to the Fick's law. In other words,atoms diffuse from a high-concentration region toward alow-concentration region in a solid. Accordingly, providing Ga inadvance in the electrode allows preventing the diffusion of Ga from thesemiconductor layer to the electrode. Adjusting the Ga content in therange of 1 at % to 25 at % enables forming the metal layer having thedesired Ga content under accurate control, as well as properlydecreasing the contact resistance of the electrode. Such arrangementenables, consequently, suppressing the deterioration in contactresistance originating from the operation of the device and thermalhistory.

The driving force of the impurity diffusion is the concentrationgradient, more strictly the chemical potential gradient of the impuritywith respect to the host substance. Accordingly, the diffusion of aspecific impurity can be suppressed by providing the same substance inadvance in a region where the impurity is supposed to diffuse. Thus, itis appropriate to provide Ga in the metal, in order to suppress thediffusion of Ga from the n-type semiconductor layer into the metallayer.

However, in case that the metal layer contains an excessive amount ofGa, the quality of the metal crystal is degraded, which leads to declinein conductivity and increase in contact resistance. The Ga contentshould therefore be restricted under a certain level.

Adjusting the Ga content of the metal layer 202 to be 25 at % or lowerpermits presence of a region solely occupied with the host metal in themetal bulk, thereby suppressing the contact resistance at a low level.It is undesirable to compose the entirety of the metal layer 202 fromthe compound containing the host metal and the Ga as component elements,because the properties become substantially different. Setting the upperlimit of the Ga content at 25 at % allows preventing forming theentirety of the metal layer with the compound.

FIG. 3 is a graph showing a simulative evaluation result of therelationship between the Ga content and the contact resistance based onthe foregoing embodiment, on the assumption that the n-GaN layer isemployed as the n-type semiconductor layer and the TiGa layer as themetal layer.

Whereas a contact resistance in the order of 10⁻⁵ Ωcm² is said to begenerally acceptable in the case of the n-side electrode, setting the Gacontent to be 25 at % or lower enables achieving the contact resistanceof 1.0×10⁻⁵ Ωcm² or lower, as shown in FIG. 3.

Although the melting point of Ga is approx. 30° C. and hence it isdifficult to handle Ga at room temperature, the metal sintered compactconstituted of a solid metal crystal with the Ga content not exceeding25 at % can be a solid. This makes it possible to employ the RFsputtering process to form the metal layer on the n-type semiconductorlayer. Also, the Ga content of 1 at % or more allows providing a desiredamount of Ga in the metal layer by the RF sputtering process orco-deposition process under accurate control.

Further, with the conventional electrode, Ga in the substrate diffusesinto the electrode through the alloying process. With the electrodeaccording to this embodiment, however, the contact resistance can besuppressed at a low level despite executing the alloying process.

Second Embodiment

This embodiment represents a semiconductor laser that includes then-side electrode according to the first embodiment. More particularly,an inner stripe GaN-based semiconductor laser shown in FIG. 4 will bereferred to. Hereunder, description will be given on the structure andworking of the semiconductor laser according to this embodiment.

FIG. 4 is a cross-sectional view showing a structure of the inner stripesemiconductor laser. The semiconductor laser of this embodiment includesa semiconductor layer structure and the metal electrode. Examples of thesemiconductor layer structure include combinations of GaN, AlGaN, orInGaN. To be more detailed, the semiconductor layer structure includesan n-type substrate 101, an n-type clad layer 102, an n-type SeparatedConfinement Heterostructure (hereinafter, SCH) layer 103, an activelayer 104, a p-type SCH layer 105, and a p-type Super Lattice Structure(hereinafter, SLS) layer 107, stacked in the mentioned order. On thep-type SLS layer 107, a p-side electrode 109 is provided. The n-sideelectrode 110 is located on the n-type clad layer 102 through the metallayer 202. The semiconductor laser according to this embodiment iscovered with an insulating layer 108, such that the layers other thanthe n-type substrate 101, the n-side electrode 110 and the p-sideelectrode 109 are kept from being exposed.

The n-type substrate 101 may be, for example, an n-GaN substrate.

The n-type clad layer 102 serves to confine electric charge and light inthe active layer 104. The n-type clad layer 102 also serves as thecontact layer that attains a low contact resistance with the n-sideelectrode 110. The n-type clad layer 102 may be, for example, an n-AlGaNlayer.

The active layer 104 is a quantum well active layer acting as a lightemitting layer. The active layer 104 may be constituted of, for example,InGaN.

An n-type current block layer 106 is provided on the p-type SCH layer105 for controlling the current route, and a lateral and upper faces arecovered with the p-type SLS layer 107. The n-type current block layer106 may be constituted of, for example, AlN.

The p-type SLS layer 107 also serves as the contact layer that attains alow contact resistance with the p-side electrode 109. The p-type SLSlayer 107 may be constituted of, for example, GaN/AlGaN.

The insulating layer 108 may be, for example, a SiO₂ layer.

FIG. 5 is a cross-sectional view for explaining the working of thesemiconductor laser according to this embodiment. Arrows in FIG. 5indicate the current route in an activated state. The current runs fromthe p-side electrode 109 to the n-side electrode 110, through the p-typeSLS layer 107, the p-type SCH layer 105, the active layer 104, then-type SCH layer 103, the n-type clad layer 102, and the n-typesubstrate 101.

Among the above, the material, composition, and thickness of the n-typesubstrate 101, the n-type clad layer 102, the n-type SCH layer 103, theactive layer 104, the p-type SCH layer 105, and the p-type SLS layer 107are associated with the magnitude of the electric resistance. Theseparameters are, however, under the restriction of controllability ofoptical distribution in the semiconductor laser, and hence cannot befreely determined so as to decrease the resistance. On the other hand,the material of the p-side electrode 109 and the n-side electrode 110may be selected so as to reduce the contact resistance, irrespective ofthe optical distribution control.

The GaN-based semiconductor laser constituted of a combination of GaN,AlGaN, or InGaN may incur an increase in working voltage while thecurrent is being supplied. A reason is that Ga from GaN diffuses in themetal electrode. The defect produced after the diffusion of Ga tends toform a hole and to act as a p-type dopant, and therefore theconcentration of electrons which are the n-type carrier decreases at theinterface between the semiconductor and the metal, so that the contactresistance increases. Also, as the number of defects increases, the voidgrows to thereby decrease the contact area between the metal and thesemiconductor, which also encourages the resistance to increase.

In the semiconductor laser according to this embodiment, however, themetal layer 202 contains Ga. Accordingly, Ga already present in themetal layer 202 blocks the diffusion of Ga from the n-type clad layer102 to the metal layer 202. Such structure prevents the deterioration ofthe contact resistance. The diffusion of Ga can also be driven by heatfrom a processing work or a plasma process, in addition to the currentsupply. Nevertheless, the foregoing structure of the semiconductor laseris effective against all such factors.

A sample of the GaN-based semiconductor laser was made up according tothis embodiment, and observation was made on the deterioration inresistance due to operation of the device and thermal history. Thevoltages upon supplying a current of 200 mA were compared between beforeand after activating the laser for 1000 hours at a temperature of 80° C.and optical output of 250 mW. As a result, while the semiconductor lasernot containing Ga in the metal layer 202 (host metal was Ti) presentedan increase of more than 0.1 V, the one containing Ga presented anincrease of less than 0.05 V. It has therefore proved that a superiorsuppressing effect of the contact resistance can be attained accordingto the latter, compared with the former.

Third Embodiment

This embodiment represents another semiconductor laser that includes then-side electrode according to the first embodiment. More particularly, aridge stripe semiconductor laser shown in FIG. 6 will be referred to.Hereunder, description will be given on the structure and working of thesemiconductor laser according to this embodiment.

FIG. 6 is a cross-sectional view showing a structure of the ridge stripesemiconductor laser. As shown therein, the ridge stripe GaN-basedsemiconductor laser includes a semiconductor layer structure and themetal electrode. Examples of the semiconductor layer structure mayinclude combinations of GaN, AlGaN, or InGaN. To be more detailed, thesemiconductor layer structure includes an n-type substrate 301, ann-type clad layer 302, an active layer 303, a p-type clad layer 304, ap-type SLS layer 305, and an insulating layer 306 stacked in thementioned order. On the p-type SLS layer 305 a p-side electrode 307 islocated so as to be in mutual contact. An n-side electrode 308 isprovided on the back face of the n-type substrate 301.

The n-type substrate 301 may be, for example, an n-GaN substrate.

The n-type clad layer 302 serves to confine electric charge and light inthe active layer 303. The n-type clad layer 302 also serves as thecontact layer that attains a low contact resistance with the n-sideelectrode 308. The n-type clad layer 302 may be, for example, an n-AlGaNlayer.

The active layer 303 is a quantum well active layer acting as a lightemitting layer. The active layer 303 may be constituted of, for example,InGaN.

The p-type clad layer 304 serves to confine electric charge and light inthe active layer 303. The p-type clad layer 304 may be, for example, ap-GaN/AlGaN layer.

The p-type SLS layer 305 also serves as the contact layer that attains alow contact resistance with the p-side electrode 307. The p-type SLSlayer 305 may be constituted of, for example, GaN/AlGaN.

The insulating layer 306 may be, for example, a SiO₂ layer.

In the semiconductor laser according to this embodiment, the n-sideelectrode 308 is disposed in contact with the n-type substrate 301. Inthe manufacturing process of the semiconductor laser, the substrate issubjected to a mechanical or chemical polishing process to thereby bemade thinner, before depositing the metal layer 202 of the n-sideelectrode 308 on the back of the substrate. Through this polishingprocess, the polished surface suffers damages and a number ofcrystalline defects are produced. Since the bonding force between theatoms constituting the crystal declines in the vicinity near the defect,Ga is more prone to diffuse from the n-type substrate 301 to the metallayer 202. In the semiconductor laser according to this embodiment,however, the metal layer 202 contains Ga. Accordingly, Ga alreadypresent in the metal layer 202 blocks the diffusion of Ga from then-type substrate 301 to the metal layer 202. Such structure can suppressthe diffusion of Ga more effectively.

Although the embodiments of the present invention have been describedreferring to the drawings, it is to be understood that those embodimentsare merely exemplary, and that various other structures may be adopted.Also, although the present invention refers to upper and lowerpositions, this is merely for convenience sake for explicitly explainingthe relationship between the constituents of the present invention, andsuch expression is not intended to determine a direction in amanufacturing process or in use, when executing the present invention.

In another exemplary embodiment, there is provided a method ofmanufacturing a semiconductor chip including

preparing an n-type semiconductor layer that contains gallium (Ga);

forming an n-side electrode on a surface of said n-type semiconductorlayer;

wherein said forming an n-side electrode includes:

depositing a Ga-containing metal layer on said surface of said n-typesemiconductor layer; and

employing a metal material having a Ga content of equal to or more than1 at % and equal to or less than 25 at %, to thereby form said metallayer.

It is apparent that the present invention is not limited to the aboveembodiment, and may be modified and changed without departing from thescope and spirit of the invention.

What is claimed is:
 1. A semiconductor laser, comprising: an n-sideelectrode to be formed on a surface of an n-type semiconductor layerthat comprises gallium (Ga), wherein said n-side electrode comprises: ametal layer having a Ga content of equal to or more than 1 at% and equalto or less than 10 at%, wherein said metal layer is disposed in contactwith said n-type semiconductor layer, and wherein a metal predominantlycomposing said metal layer is Ti.
 2. The semiconductor laser accordingto claim 1, wherein said metal layer includes a metal material having aGa content of equal to or more than 1 at% and equal to or less than 10at%.
 3. The semiconductor laser according to claim 1, wherein saidn-type semiconductor layer includes a III-V compound.
 4. Thesemiconductor laser according to claim 1, wherein said metalpredominantly composing said metal layer has a work function of equal toor less than 5 eV.
 5. The semiconductor laser according to claim 1,wherein said n-type semiconductor layer is an n-type nitridesemiconductor layer.
 6. The semiconductor laser according to claim 1,wherein said n-side electrode further comprises a gold-plating layerdisposed above the metal layer.
 7. The semiconductor laser according toclaim 6, wherein said n-side electrode further comprises a barrier layerdisposed between the gold-plating layer and the metal layer.
 8. Thesemiconductor laser according to claim 6, wherein said n-side electrodefurther comprises a gold layer and a platinum layer disposed between thegold-plating layer and the metal layer.
 9. The semiconductor laseraccording to claim 1, wherein said n-type semiconductor layer comprisesTi₃Ga or Ti₂Ga.
 10. A recording and reproducing apparatus, comprising asemiconductor laser according to claim l.
 11. The semiconductor laseraccording to claim 1, wherein a thickness of the metal layer is 100 Å ormore.